1. Field of the Invention
The present invention relates to a method of forming a transparent conductive film and an apparatus for forming the same.
2. Description of the Related Art
A sputtering method is widely employed as a method of forming a transparent conductive film such as an ITO (indium-tin oxide) film on an insulating substrate such as a glass substrate.
According to the sputtering method, a gas of a Group VIII element of the periodic table (e.g., argon gas) and oxygen gas are supplied to a sputtering chamber at a predetermined mixing ratio. Sputtering is performed in the gas atmosphere while the gas mixture continuously flows in the sputtering chamber and a current supplied to a target material is kept constant, thereby depositing a transparent conductive film on the surface of the substrate.
Apparatuses for forming transparent conductive films by the sputtering method, that is, sputtering apparatuses, are classified into a batch type sputtering apparatus for performing sputtering after the transparent substrate is fixed in the sputtering chamber and an in-line type sputtering apparatus for performing continuous sputtering while the transparent substrates are sequentially fed to the sputtering chamber. The in-line sputtering apparatus is excellent in efficient formation of a large number of substrates.
In the conventional sputtering apparatus described above, even if sputtering temperature, sputtering current, argon gas flow rate, and oxygen gas flow rate are kept constant, resistivities of transparent conductive films formed on substrates vary in different cycles of the sputtering apparatus. As shown in FIG. 1, which shows a relationship between the resistivity and the oxygen flow rate when an ITO transparent conductive film is formed on a glass substrate by using an in-line sputtering apparatus, resistivities of the respective transparent conductive films continuously formed by the first operation cycle under the conditions that the sputtering temperature is 160.degree. C., and the sputtering current and argon gas flow rate are kept constant while the oxygen gas flow rate is changed are represented by points on curve A. However, resistivities of the respective conductive films continuously formed by a second operation cycle under the condition that sputtering is started after the pressure in a pressure vessel of the sputtering apparatus is restored to atmospheric pressure are represented by curve B. That is, even if the identical sputtering conditions are set, the resistivities of the transparent conductive films are different in units of operation cycles of the sputtering apparatus, thus resulting in poor reproducibility.
When transparent conductive films are continuously formed on a large number of substrates by using an inline sputtering apparatus, resistivities of the transparent conductive files are increased when the number of substrates to be processed is large As a result, transparent conductive films having equal resistivities cannot be formed on substrates. Even if identical film formation conditions are used in an in-line sputtering apparatus, thicknesses of the transparent conductive films are decreased upon an increase in the number of substrates to be processed. As shown in FIG. 2, which shows a relationship between total discharge time and the thicknesses of the transparent conductive films under identical film formation conditions, when the number of substrates to be processed and the total discharge time are increased, the thickness of the transparent conductive film formed within a period of time assigned to one substrate is greatly decreased in the initial period and hen gradually decreased thereafter. Spectral distributions of the transparent conductive films are different from each other in units of substrates, as shown in FIG. 3, which shows the spectral distribution of light transmitted through three conductive films having thicknesses of 1,970 .ANG., 1,630 .ANG., and 1,280 .ANG..